Field of the Invention
The invention relates to a printing unit cylinder protected against corrosion, i.e., a corrosion-proof printing unit cylinder, and a method of protecting the printing unit cylinder against corrosion.
Printing unit cylinders installed in modern printing presses, especially rubber blanket cylinders and plate cylinders of web or sheet-fed rotary offset printing presses, are typically coated with protective layers in order to prevent corrosion of the cylinder surface. Simple anti-corrosion layers for gray cast iron or steel cylinders are created by phosphating and subsequent oiling of the cylinder jacket face. It has also become known to create high-quality anti-corrosion layers for printing unit cylinders by electroplating with zinc, nickel and chromium, as described in U.S. Pat. No. 4,643,095, for example, or to melt onto the plate cylinder, with the aid of a laser (German Patent 36 08 286), similarly thin metal layers of chromium, nickel, and so forth. Other metal layers, such as high-quality or stainless steel, are applied to the printing cylinder by plasma coating in an argon atmosphere, for example as taught by the published Japanese Patent Document JP 4-238034.
Besides the fact that these conventional methods are relatively complicated and consequently expensive, they often also require post-machining or after-treatment of the cylinder jacket face in order to ensure that dimensional deviations in the cylinder formed after the coating process remain within a tolerance, typically 5 .mu.m, prescribed for the cylinders.
From both the published non-prosecuted European Patent Application EP 0 583 543 A1 and U.S. Pat. No. 4,643,095, it has also become known to provide the impression cylinder, and not the plate cylinder or rubber blanket cylinder, in the printing unit of a rotary printing press with a layer which, however, is a relatively thick, namely from 0.4 to 1 mm thick, layer of plastic material formed of polyurethane.
The intent in the foregoing references, however, is not to protect the impression cylinder against corrosion but rather to utilize the resilient and elastic properties, respectively, of such a layer in the direct printing process described therein in order to optimize the printing or reproduction performance.
In the published non-prosecuted Japanese Patent Application JP-61-79697, a dampening roller for the dampening unit of an offset printing press is described which carries a hydrophilic layer formed of a mixture of polyurethane and quartz powder and having a thickness of between 50 .mu.m and 150 .mu.m. Besides the fact that neither a printing cylinder nor an anti-corrosion layer is involved in this publication, the surface of the dampening roller described therein is also ground after it has been coated.
Generally, conventional coatings of such plastic materials as polyamides (for example, known under the trade name Rilsan) applied to rollers in the inking or dampening unit of printing presses are not especially well suited as anti-corrosion layers, because these layers adhere rather poorly to the surface of the cylinders and are therefore shrunken relatively thickly onto the cylinder surface so that post-machining or after-treatment of the cylinder surface is then required. Such a shrinkage package is moreover possible only for completely closed cylinder surfaces.